Image reading apparatuses for optically scanning an original image have been broadly used, for example, in copying machines and have also been used as image scanners coupled with computers or the like.
The image reading apparatus generally includes first and second sliders. The first slider is reciprocatable and carries a lighting device for lighting an original document laid at a predetermined position and a reflection mirror for leading the image light beams reflected by the original document in a predetermined direction. The second slider carries reflection mirrors for leading the image light beams coming from the reflection mirror on the first slider in a predetermined direction, and can reciprocate in the same direction as the first slider.
As the lighting device carried on the first slider, a fluorescent lamp is generally used. The power circuit or driving circuit for the fluorescent lamp is generally arranged at a deep position (i.e., rear position) in the apparatus. One of the reasons for this arrangement is as follows. A sufficient space is left at the rear position in the apparatus, and an inlet of AC power supply of the apparatus is also located at the rear position in the apparatus in many cases. Therefore, the circuit located at the rear position does not require complicated wiring.
The image reading apparatus such as an image scanner and a copying machine equipped with the image reading apparatus are generally provided with an operation panel having key switches and a display or the like for displaying various kinds of information. A LCD is now used as this display in many cases, and the operation panel is usually arranged at the front side of the apparatus for easy operation by operators.
In the above image reading apparatus, and particularly in the image reading apparatus, for example, equipped with the foregoing two sliders carrying the optical devices or parts, an image on an original document laid at a predetermined position (e.g., on an original document table glass) is read in such a manner that a lighting device carried on the first slider is turned on, the first slider is driven at a predetermined speed in the predetermined direction parallel with the original document, and concurrently the second slider is driven in the same direction as the first slider at half the speed of the first slider. Thereby, the original document is scanned entirely and optically. During this scanning, light beams, which are issued from the lighting device and are reflected by the original image, are lead in the predetermined direction by the mirrors on the first and second sliders. An image sensor such as a CCD is arranged at a position to which the image light beams are led by the mirrors. The image sensor can read the original image. Alternatively, if the image reading apparatus is arranged, for example, in an analog copying machine, the original image light beams led by the above mirrors are led by another mirror or the like, if necessary, to a photosensitive member for forming an electrostatic latent image corresponding to the original image.
As described above, the optical parts for optically scanning and reading the original image are carried on the sliders, and, for example, linear motors may be used for linearly reciprocating the sliders.
The linear motors can be classified into various types such as a linear DC motor, a linear pulse motor and a linear induction motor, which have distinctive features suitable to use in various kinds of equipments for linearly moving objects, respectively.
For example, in linear induction motor taught by U.S. Pat. No. 4,512,385, an armature coil group formed of a plurality of armature coils is fitted around a shaft member provided with a field magnet. The shaft member forms a movable piece, and the armature coil group forms a stator. In this linear induction motor, the armature coil group of the stator is covered and protected by a cylindrical cover.
FIG. 25 is a schematic side view of an example of a linear motor. This linear motor has a field magnet 911' in a shaft-like form on which magnetic poles of N- and S-types are arranged linearly and alternately to each other, and an armature coil 921' fitted around the field magnet 911'. The armature coil 921' is carried at an inner periphery of a yoke 922' made of a ferromagnetic material in a hollow cylindrical form. Slide bearings 923' fitted around the field magnet 911' are arranged at opposite open ends of the yoke 922', so that the armature coil 921' and the yoke 922' can smoothly move along the field magnet 911'. In this linear motor, the field magnet 911' functions as a stator 91', and the coil 921' and yoke 922' function as a movable piece 92' reciprocatable along the stator 91'. When the armature coil 921' is energized, the movable piece 92' generates a driving force and moves along the stator 91' owing to an interaction with respect to a magnetic field produced by the field magnet 911'. Owing to provision of the yoke 922' made of a ferromagnetic material, the magnetic field, which is produced by the field magnet 911' at the position opposed to the yoke 922', is liable to form a magnetic loop through the yoke 922', so that the intensity of the magnetic field acting on the armature coil 921' inside the yoke 922' is larger than that in the case where the yoke is not employed. Therefore, the linear motor can generate a larger driving force. Thus, owing to the yoke 922', the magnetic field formed by the field magnet 911' can efficiently act on the coil 921'.
In the linear motor of the foregoing type which includes the shaft-like stator having the field magnet extending in the predetermined direction and the movable piece having the armature coil fitted around the field magnet, the shaft-like stator itself can be utilized also as a guide member for the movable piece, so that the structure can be simplified. For this and other reasons, the linear motors of the above type have been broadly used for linearly moving objects in fields of office automation equipments such as a copying machine, a printer and an image scanner as well as factory automation equipments such as an X-Y table and an object transporting device, and optical equipments such as a camera.
The linear motor described above usually includes a linear encoder for detecting a position of the movable piece or the like. Likewise, the image reading apparatus provided with the reciprocatable slider usually includes a linear encoder for detecting a position of each slider or the like. The encoder may be utilized for controlling a position and/or a speed in addition to detection of the position of a moving object such as the movable piece or sliders. An optical type and a magnetic type of the linear encoder have been known. For example, the magnetic encoder is generally provided with a magnetic encoder scale having S- and N-type magnetic poles arranged alternately with a fine pitch, and a magnetism detecting element (e.g., an MR element which is a magnetoelectric resistance element) for detecting the magnetic field formed by the encoder scale. The encoder scale is stationarily arranged parallel to the moving direction of the movable object such as the movable piece or the slider. The magnetism detecting element is opposed to the encoder scale, and is disposed on the movable object for movement together with the movable object. The magnetism detecting element is usually used together with an amplifier circuit for amplifying an extremely weak detection signal of the magnetism detecting element, and a detecting circuit including a circuit or the like for digitizing the signal.
However, in the case where the foregoing magnetic encoder is used as a linear encoder for detecting a position of the movable object such as the movable piece or the slider, the magnetism detection element (e.g., MR element) and the detection circuit are liable to be affected by noises, because it processes extremely weak signals and analog signals. In the foregoing image reading apparatus, noise sources of the noises may be the fluorescent lamp turn-on circuit and the LCD. If the fluorescent lamp and/or the LCD are arranged near the magnetism detecting element and/or the detection circuit, these element and circuit may be affected by noises, and therefore a problem may arise in the position detection. When a problem arises in position detection, image reading can not be precisely performed, and the slider may run away out of control or collide with another member. A similar problem may occur in the linear motor provided with the magnetic encoder. When a magnetic field is present near the linear motor, the magnetic field may cause problems in position detection by the magnetism detecting element and/or the detecting circuit.
In addition to the foregoing, in the linear induction motor taught by U. S. Pat. No. 4,562,385, although the stator, i.e., armature coil group is protected by the cylindrical cover, a protection cover is not provided for the movable shaft member. Therefore the motor suffers from such a problem that dust or the like may adhere onto the shaft member and thereby may impede sliding on the stator.
A similar problem may arise even in a shaft-type linear motor, in which a shaft member provided with a field magnet forms a stator and a member having an armature coil and fitted around the stator forms a movable piece in contrast to the above linear induction motor.
For example, in the linear motor shown in FIG. 25, dust or the like may adhere onto the stator 91' in a shaft form, in which case a sliding resistance varies during sliding of the movable piece 92' along the stator 91', and the movable piece 92' cannot smoothly move along the stator 91'.
The linear motor shown in FIG. 25 also suffers from another problem. When the movable piece 92' moves relatively to the stator 91', a load varies due to variation in a magnetic attractive force which is exerted by the field magnet 911' acting on the end of the yoke 922' of the movable piece 92' so that cogging of the movable piece 92' occurs, and thus smooth movement of the movable piece 92' is prevented.